Remove the unused utilities.parsing.flatten_dict

[carmocca]

What does this PR do?

Removes unused code

cc @justusschock @awaelchli @Borda

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu (macOS-11, lightning, 3.8, 1.11)	success	✅
pl-cpu (macOS-11, lightning, 3.9, 1.12)	success	✅
pl-cpu (macOS-11, lightning, 3.10, 1.13)	success	✅
pl-cpu (macOS-11, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.9, 1.11)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.10, 1.12)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.10, 1.13)	success	✅
pl-cpu (ubuntu-20.04, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (windows-2022, lightning, 3.9, 1.11)	success	✅
pl-cpu (windows-2022, lightning, 3.10, 1.12)	success	✅
pl-cpu (windows-2022, lightning, 3.10, 1.13)	success	✅
pl-cpu (windows-2022, lightning, 3.8, 1.11, oldest)	success	✅
pl-cpu (macOS-11, pytorch, 3.8, 1.13)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.8, 1.13)	success	✅
pl-cpu (windows-2022, pytorch, 3.8, 1.13)	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py, tests/tests_pytorch/utilities/test_parsing.py.

🟢 pytorch_lightning: Azure GPU

Check ID	Status
pytorch-lightning (GPUs)	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py, tests/tests_pytorch/utilities/test_parsing.py.

🟢 pytorch_lightning: Azure HPU

Check ID	Status
pytorch-lightning (HPUs)	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py, tests/tests_pytorch/utilities/test_parsing.py.

🟢 pytorch_lightning: Azure IPU

Check ID	Status
pytorch-lightning (IPUs)	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py, tests/tests_pytorch/utilities/test_parsing.py.

🟢 pytorch_lightning: Docs

Check ID	Status
make-doctest (pytorch)	success	✅
make-html (pytorch)	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py.

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.8)	success	✅
install-pkg (ubuntu-22.04, app, 3.10)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.8)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.10)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.8)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.10)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.8)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.10)	success	✅
install-pkg (ubuntu-22.04, notset, 3.8)	success	✅
install-pkg (ubuntu-22.04, notset, 3.10)	success	✅
install-pkg (macOS-12, app, 3.8)	success	✅
install-pkg (macOS-12, app, 3.10)	success	✅
install-pkg (macOS-12, fabric, 3.8)	success	✅
install-pkg (macOS-12, fabric, 3.10)	success	✅
install-pkg (macOS-12, pytorch, 3.8)	success	✅
install-pkg (macOS-12, pytorch, 3.10)	success	✅
install-pkg (macOS-12, lightning, 3.8)	success	✅
install-pkg (macOS-12, lightning, 3.10)	success	✅
install-pkg (macOS-12, notset, 3.8)	success	✅
install-pkg (macOS-12, notset, 3.10)	success	✅
install-pkg (windows-2022, app, 3.8)	success	✅
install-pkg (windows-2022, app, 3.10)	success	✅
install-pkg (windows-2022, fabric, 3.8)	success	✅
install-pkg (windows-2022, fabric, 3.10)	success	✅
install-pkg (windows-2022, pytorch, 3.8)	success	✅
install-pkg (windows-2022, pytorch, 3.10)	success	✅
install-pkg (windows-2022, lightning, 3.8)	success	✅
install-pkg (windows-2022, lightning, 3.10)	success	✅
install-pkg (windows-2022, notset, 3.8)	success	✅
install-pkg (windows-2022, notset, 3.10)	success	✅

These checks are required after the changes to src/lightning/pytorch/utilities/__init__.py, src/lightning/pytorch/utilities/parsing.py.

🟢 link-check

Check ID	Status
markdown-link-check	success	✅

These checks are required after the changes to src/lightning/pytorch/CHANGELOG.md.

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